The invention concerns the drive system of an agricultural vehicle, with a hydraulic motor that drives elements in contact with the ground for the propulsion of the vehicle, in particular wheels and/or crawler chains, with an intervening gearbox, where the hydraulic motor is able to act as a brake upon the elements in contact with the ground.
In agricultural vehicles and front harvesting attachments, such as cutter heads and corn pickers, a tendency can be see towards ever more powerful, larger and therefore heavier units. For reasons of safety of operation and of traffic, there s also the necessity to dimension the brakes of the vehicles correspondingly. In order to avoid having to increase the size and thereby the cost of the conventional friction brakes, that are, as a rule, disk or drum brakes, and act mechanically on the axles of the wheels, there is also the possibility of utilizing the braking effect of a hydrostatic drive of the vehicle, whose swash plate is brought into the null position.
In a publication (VkB1. official part, volume 21-1998, dated Oct. 5, 1998, page 1226 and following) guidelines are forth for brake systems of vehicles with hydrostatic drives in which it has been established that such vehicles are admitted to traffic over roads only if this drive cannot be uncoupled during the operation and operates as a hydrostatic brake system or is a component of at least one brake system. It is not disclosed how the uncoupling can be prevented.
EP 0819562 A describes a gearbox shifting system for a harvesting machine in which the user can push a button associated with the desired gear ratio. If the button is pushed, a mechanical brake, independent of the gearbox, is applied automatically by a control arrangement, which brings the harvesting machine to a stop. If the harvesting machine is stopped, which can be determined on the basis of the expiration of a pre-determined time interval that is required in the general case for the braking of the harvesting machine, or by means of a speed sensor, then the shifting process is performed. The reason for this is the fact that the hydrostatic gearbox of the harvesting machine can only be shifted when the machine is stopped. The disadvantage is the high cost.
U.S. Pat. No. 4,572,340 describes a blocking arrangement for an automatic transmission, which permits a shift between a neutral, forward and reverse position of the selector lever of the automatic transmission only if the service brakes of the vehicle are applied. The blocking arrangement locks the selector lever for such a time until the brakes are applied.
The problem underlying the invention is seen as that of creating an agricultural vehicle with a hydrostatic traction drive that is safe in traffic and low in cost.
According to the present invention there is provided an improved drive system for the ground-engaging propulsion elements of an agricultural vehicle wherein the drive system embodies a hydrostatic transmission coupled to a gear transmission.
An object of the invention is to provide a mixed hydrostatic and gear transmission that uses the hydrostatic transmission for braking the ground wheels with a simple inexpensive way of controlling the drive system so that gear ratio changes in the gear transmission cannot be done unless the vehicle is at a standstill.
A more specific object of the invention is to provide a drive system as set forth in the previous object wherein the shifting assembly for the gear transmission includes a device which blocks the shifting operation if the propulsion wheels are rotating.
Yet a more specific object is to provide a drive system as set forth in the immediately preceding object wherein the device which blocks the shifting operation includes a controller which is responsive to the pressure developed by a lubrication pump for completing a circuit for energizing a solenoid that positions a locking pin in a lock position any time the propulsion wheels are being driven by the hydrostatic transmission or when they are coasting.
These and other objects of the invention will become apparent from a reading of the ensuing description together with the appended drawings.